The general concept of Raman signal enhancement near a sub-micrometer-sized metal particle disposed at a tip has been proposed and realized. The presence of a metal particle at the tip enhances the strength of an electrical field of light, thereby inducing enhancement of that portion of the total Raman signal within 10 to 20 nm of the particle's surface. This portion of the total Raman signal adjacent to the surface of the metal particle is referred to herein as the near-field signal and the portion of the total Raman signal outside this region is the far-field signal. In this way, the spatial resolution of Raman spectroscopy can be improved, in one embodiment, to approximately the radius of the tip provided with the metal particle (approximately 10 nm in one instance). This is far below the traditional diffraction limit of light, which is about 500 nm. This idea has recently been embodied in the form of tip enhanced Raman spectroscopy (TERS), which couples a Raman spectrometer to an atomic force microscope (AFM) with a metallic or metal-coated tip.
A TERS signal has two main components: one coming from the near vicinity of the tip (the near-field signal), and another coming from the rest of the laser illuminated area, this other component being referred to herein as the far-field signal. To obtain effective results from TERS, the contrast ratio of the near-field signal strength to the far-field signal strength must be high—i.e., the near-field signal should be much stronger than the far field signal. Raman mapping (analysis of chemical structure, composition, stresses, etc.) with extreme lateral resolution can be achieved if the contrast ratio is high without further data manipulation. Known reported TERS measurements on silicon (Si) exhibit an approximate 30-50% increase in the total TERS signal over the far-field Raman signal alone, resulting in a contrast ratio of near-field to far-field signal about 0.3 to 0.5. This small contrast is insufficient for scanning TERS and will require subtraction of the far-field signal to achieve useful results. Such corrections will introduce significant uncertainty, strongly decrease the accuracy of the measurements, and drastically increase measurement time. The higher the contrast the better, but for practical purposes, a contrast ratio of at least about 3, or even higher, is desirable.
Variations on the TERS technique include bottom, top, and side illumination geometries, where the light source is positioned in the respective orientations relative to the sample to be mapped using TERS. The use of top-illumination geometry with a depolarization configuration has been proposed to increase TERS contrast for mapping Si. Since Raman-scattered light from Si is strongly polarized (there is no change in polarization of the illumination light upon being scattered), the far-field Raman signal can be suppressed by using a depolarizing optical configuration, i.e., by measuring the scattered light spectra with polarization perpendicular to the polarization of the illumination beam. In such a case, the metal particles at the tip depolarize the illumination light and induce depolarized scattering of Si in the vicinity of the particle. Therefore, the total depolarized TERS signal is mainly due to the near-field contribution and the far-field signal is strongly suppressed. The resulting contrast between the depolarized TERS and the normal Raman signal is greater than that from TERS without a depolarization configuration. However, the proposed top-illumination geometry poses several problems to commercialization. First, the tip (an Ag particle at the end of a quartz tip) and the sample are submerged in glycerol to eliminate scattering and shadowing from the quartz tip. It is neither simple nor convenient to work in this configuration. Second, the silver particle, which has an approximately 50 to 100 nm radius, lies between the Si and the signal collecting optics. This geometry has a number of disadvantages: (i) it restricts the near-field Raman signal to the edges of the particle; (ii) it limits lateral resolution to at least the size of the particle or even larger (approximately 100 nm); (iii) the maximum enhanced signal from the Si surface closest to the particle is lost (shadowed by the particle).
Accordingly, there is a need in the art for a TERS method and system that can achieve a high contrast ratio of at least 3, with improved lateral resolution (in one embodiment, about 10 to about 30 nanometers). The system should be able to map both transparent and non-transparent materials, and should minimize the complexity of performing TERS mapping.